Human T lymphotropic virus type 1 (hereinafter referred to as “HTLV-1”) is a retrovirus that is integrated into the genomic DNA of CD4-positive T lymphocytes and infectiously propagated in vive and between individuals. Tumorigenesis of HTLV-1-infected cells causes adult T cell leukemia (ATL), and HTLV-1-infected cells infiltrate into the spinal cord, thereby inducing HTLV-1-associated myelopathy (HAM/TSP: HTLV-1-associated myelopathy/tropical spastic paraparesis, hereinafter also referred to as “HAM”) which causes spastic spinal paralysis and dysuria. Further, infiltration of HTLV-1-infected cells into the eye may cause HTLV-1-associated uveitis (HU).
The pathogenic mechanism of HAM among various diseases caused by HTLV-1 described above is unknown except that HTLV-1-infected cells infiltrate into the spinal cord and trigger inflammation. At present, no curative therapy has been established for HAM, and treatment for suppressing inflammation is performed depending on the rate of disease progression and the intensity of inflammation. In a case in which the inflammation is severe, symptoms are likely to progress. In such case, treatment such as steroid therapy or interferon-alpha injection therapy is selected to suppress inflammation and prevent the spinal cord from being destroyed. Specifically, it has been confirmed that oral administration of a corticosteroid (predonisolone) as an anti-inflammatory agent causes the proviral load to decrease to some extent, which is an effect of improving symptoms. However, in many cases, the above treatment cannot achieve complete cure, and there are problems such as a rebound phenomenon due to discontinuation of drugs. The treatment method is further problematic due to side effects such as steroid diabetes, osteoporosis, and immunosuppression.
As described above, there has been a need for therapeutic agents capable of achieving complete cure of HAM caused by HTLV-1. For example, Patent Literature 1 discloses a therapeutic agent for HTLV-1-associated myelopathy including an anti-human CXCL10 antibody that specifically binds to human CXCL10 or a fragment of the antibody.